1. Field of the Invention
The invention relates to a process for coating the surface of an organic polymer to protect it against erosion by oxygen atoms. More specifically, the invention relates to such a process which includes the step of depositing a thin protective barrier film on the surface.
2. Description of Prior Art
All matter on the earth's surface, both living and inanimate, is protected by the atmosphere. This is not the case in outer space, where the atmosphere's density decreases rapidly with distance above the earth's surface; this decreases its ability to attenuate such harmful effects as, for example, energetic radiation (e.g. ultraviolet, UV). The insufficiency of life-sustaining molecular oxygen gas (due to the partial vacuum) and bombardment by short wavelength (high energy) electromagnetic radiation and by charged particles (electrons and ions) render any form of life impossible even a short distance (tens or hundreds of kilometers) above the earth's surface. The effects of charged particles are further enhanced in the so-called magnetosphere, some hundreds to thousands of kilometers above the earth's surface, where the earth's magnetic field traps and concentrates charged particles. The well-known van Allen radiation belts are related to this effect, as are such visible phenomena as the Aurora Borealis.
The present invention is primarily concerned with man-made objects (satellites) in earth orbit, and more specifically with those in "low earth orbit" (LEO), namely at altitudes of about 150 to 700 km. At these altitudes the major atmospheric constituent is atomic oxygen, O, which occurs as a result of UV-induced dissociation of molecular oxygen, O.sub.2. Experience on shuttle flights has shown that even a short-term exposure to this environment can have harmful effects on spacecraft surfaces, particularly if these are composed of or covered with organic (polymeric) materials. Organic polymers are important to the design of present and future generations of spacecraft, as they are used in advanced composites for structural members, and for thermal protection in the form of films and coatings. For instance, at the altitude of the space shuttle (about 250 km), the atomic oxygen density (.perspectiveto.10.sup.-19 atoms cm.sup.-3) is not very high, even though its concentration exceeds that of the next most abundant atmospheric constituent, molecular nitrogen, N.sub.2. However, due to the high orbital velocity (about 8 km/sec) the O atom flux is quite high, about 10.sup.15 atoms/cm.sup.2 sec. Furthermore, this high velocity corresponds to collisions with highly energetic (about 5.0 eV) atomic projectiles. The observed result has been that exposed polymer surfaces are eroded by the chemically highly reactive O atoms, and that they suffer appreciable roughening and loss of mass. This, in turn, leads to irreversible degradation of the physical characteristics for which the surface or structural member were designed.
Erosion of organic materials by atomic oxygen in space has been described by various authors, for example by D. G. Zimcik and C. R. Maag, "Results of Apparent Atomic Oxygen Reactions with Spacecraft Materials during Shuttle Flight STS-41G", presented at AIAA "Shuttle Environment and Operations II Conference", Houston, Tex., November 1985, paper No. 85/7020.
Additional damage to insulating spacecraft materials such as organic polymers may result from charge buildup due to impinging electrons and ions. As described, for example, by K. G. Balmain and W. Hirt in IEEE Transactions on Electrical Insulation, Vol. EI-18, pages 498 to 503, 1983, charging by energetic electrons in space can lead to surface discharge arcs or flashover, which can permanently damage the polymer's structure and integrity. Furthermore, electrical transients associated with these discharge events can perturb or destroy electronic circuitry aboard the space vehicle.
Several authors have suggested remedies to the problems described above. For example, B. A. Banks et al. ("Ion Beam Sputter Deposited Thin Film Coatings for Protection of Spacecraft Polymers in Low Earth Orbit" presented at the 23rd AIAA Aerospace Science Meeting, Reno, Nev., Jan. 14-17, 1985) J. S. Sovey et al (U.S. Pat. No. 4,664,980) describe the use of ion beam sputter deposited thin coatings as oxidation barriers. Their coatings are composed of metal oxides (from the group of Si, Al, In, Sn) and up to 20 percent of a molecularly mixed flexible polymer (such as polytetrafluoroethylene), the purpose of the latter--according to the authors--being to provide the required flexibility. It was found, in both ground-based laboratory and in-space tests, that these thin protective coatings can act as barriers against oxidative erosion of the underlying polymer. Other authors have also shown thin layers of pure Teflon and sputtered or vapor deposited indium-tin oxide (ITO) to possess similar protective properties against attack by atomic oxygen.